[Coco] CoCo 3's CoCo 1/2 mode

[Arthur Flexser]

I found this at http://www.roust-it.dk/coco/Lomont_CoCoHardware_2.pdf
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To use CoCo 3 graphics, the COCO bit must be set to zero. When using CoCo
1/2 resolutions, the bit is set to 1. RSDOS typically sets the INIT0
register [$FF90] to 196 in CoCo 2 resolutions and 68 when using CoCo 3
graphics modes.
-----------

So, it would seem that Bit 7 of $FF90 (the "COCO bit") is just something
that has to be set appropriately by the user, or by the Basic ROM graphics
routines, depending on whether the CoCo 3 or the CoCo 1/2 graphics modes
are being employed.  It's not a general compatibility switch.  At startup,
the CoCo 3 defaults to CoCo 1/2 graphics resolution, so $FF90 bit 7 starts
out set to 1.

Art

On Sun, Aug 22, 2021 at 11:38 PM M. David Johnson <mdj at bds-soft.com> wrote:

> I read that when Bit 7 of &HFF90 (cf.
> http://www.cococommunity.net/socks-gime-register-reference/ ) is set, the
> CoCo 3 is then in CoCo 1/2 mode.
>
>
>
> I don't have a physical CoCo 3 to check that on (I only have a CoCo 2).
>
>
>
> But in VCC 2.1.0d I notice that, immediately on startup, &HFF90 contains
> &HCC (1100 1100 binary).
>
>
>
> So it's already in CoCo 1/2 mode? Am I missing something?
>
>
>
> --
>
> M. David Johnson
>
> mdj at bds-soft.com
>
>
>
>
>
>
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